+e CO 
442 
NATURE 
theme, since all know that fuel is coal drawn from the earth 
from deposits, with which this country especially has been 
bountifully supplied ; why disturb our plain understanding by 
scientific definitions which will neither reduce the cost of coal, 
nor make it last longer on our domestic hearth ? 
Yet I must claim your patience for a little, lest, if we do not 
first agree upon the essential nature of fuel, we may afterwards 
be at variance in discussing its origin and its uses, the latter at 
any rate being of practical interest, and a subject worthy of 
your most attentive consideration. 
Fuel, then, in the ordinary acceptation of the term, is carbo- 
naceous matter, which may be in the solid, the liquid, or in the 
gaseous condition, and which, in combining with oxygen, gives 
rise to the phenomenon of heat. Commonly speaking, this 
development of heat is accompanied by flame, because the sub- 
stance produced in combustion is gaseous. In burning coal, for 
instance, on a fire-grate, the oxygen of the atmosphere enters 
into combination with the solid carbon of the coal and produces 
carbonic acid—a gas which enters the atmosphere, of which it 
forms a necessary constituent, since without it the growth of trees 
and other plants would be impossible. But combustion is not 
necessarily accompanied by flame, or even by a display of in- 
tense heat. The metal magnesium burns with a great display of 
light and heat, but without flame, because the product of com- 
bustion is not a gas but a solid, viz. oxide of magnesia. Again, 
metallic iron, if in a finely divided state, ignites when exposed to 
the atmosphere, giving rise to the phenomena of heat and light 
without flame, because the result of combustion is iron oxide or 
rust; but the same iron, if presented to the atmosphere—more 
especially to a damp atmosphere—in a solid condition, does not 
ignite, but is nevertheless gradually converted into metallic oxide 
or rust as before. : 
Here, then, we have combination without the phenomena 
either of flame or light; but by careful experiment we should 
find that heat is nevertheless produced, and that the amount of 
heat so produced precisely equals that obtained more rapidly in 
exposing spongy iron to the action of oxygen. Only, in the latter 
case the heat is developed by slow degrees, and is dispersed as 
soon as produced ; whereas in the former the rate of production 
exceeds the rate of dispersion, and heat, therefore, accumulates 
to the extent of raising the mass to redness. It is evident from 
these experiments that we have to widen our conception, and 
call fuel ‘t any substance which is capable of entering into com- 
bination with another substance, and in so doing gives rise to the 
phenomenon of heat.” 
In thus defining fuel, it might appear at first sight that we 
should find upon our earth a great variety, and an inexhaustible 
supply of substances that might be ranged under this head ; but 
a closer investigation will soon reveal the fact that its supply is, 
comparatively speaking, extremely limited. 
In looking at the solid crust of the earth, we find it to be 
composed for the most part of siliceous, calcareous, and magne- 
sian rock; the former, silica, consisting of the metal silicon 
combined with oxygen, and is therefore not fuel, but rather a 
burnt substance which has parted with its heat of combustion 
ages ago; the second limestone, being carbonate of lime, or the 
combination of two substances, viz., oxide of calcium and 
carbonic acid, both of which are essentially products of com- 
bustion, the one of the metal calcium and the other of carbon ; 
and the third, magnesia, being the substance magnesium, which 
I have just burnt before you, and which, further combined 
with lime, constitutes dolomite rock, of which the Alps are 
mainly composed. All the commoner metals, such as iron, 
zine, tin, alumina, sodivm, &c., we find in nature in an oxidised 
or burnt condition ; and the only metallic substances that have 
resisted the intense oxidising action that must have prevailed at 
one period of the earth's creation are the so-called precious metals, 
gold, platinum, iridium, and to some extent also silver and cop- 
per. But what about the oceans of water, which have occasionally 
been cited as representing a vast store of heat-producing power 
ready for our use when coal shall be exhausted? Not many 
months ago, indeed, on the occasion of a water-gas company 
being formed, statements to this effect could be seen in some of 
our leading papers. Nothing, however, could be more fallacious. 
When hydrogen burns, doubtless a great development of heat 
ensues, but water is already the result of this combustion (which 
took place upon the globe before the ocean was formed), and the 
separation of these two substances would take precisely the same 
amount of heat as was originally produced in the combustion. 
It will thus be scea that both the solid and fluid constituents of 
our earth, with the exception of cal, of naphtha (which is a | 
mere modification of coal), and the precious metals, are products — 
of combustion, and therefore the very reverse of fuel. Our earth 
may indeed be looked upon as ‘‘a ball of cinder, rolling eternally 
through space,” but happily in company with another celestial 
body—the sun—whose glorious beams are the physical cause of 
everything that moves and lives, or that has the power within 
itself of imparting life, heat, or motion. The invigorating influ- 
ence is made perceptible to our senses in the form of heat, but it 
is fair to ask, what is heat, that it should be capable of coming to 
us from the sun, and of being treasured up in our fuel deposits 
both below and on the surface of the earth ? 
If this inquiry had been put to me thirty years ago, I should 
have been much perplexed. By reference to books on Physical 
Science, I should have learnt that heat was a subtle fluid which, 
somehow or other, had taken up its residence in the fuel, and 
which, upon ignition of the latter, was sallying forth either to 
vanish or to abide elsewhere ; but I should not have been able 
to associate the two ideas of combustion and development of heat 
by any intelligible principle in nature, or to suggest any process 
by which it could have been derived from the sun and petrified, 
or, as the empty phrase ran, rendered latent in the fuel. 
It is by the labours of Meyer, Joule, Clausius, Ranken, and 
other modern physicists, that, we are enabled to give to heat its 
true significance. s 
Heat, according to the “dynamical theory,” is neither more 
nor less than motion amongst the particles of the substance 
heated, which motion, when once produced, may be changed in 
its direction and its nature, and thus be converted into mechanical 
effect, expressible in foot pounds, or horse power. By intensify- 
ing this motion among the particles, it is made eyident to our 
visual organ by the emanation of light, which again is neither 
more nor less than yibratory motion imparted by the ignited 
substance to the medium separating us from the same. According 
to this theory, which constitutes one of the most important ad- 
vances in science of the present century, heat, light, electricity, 
and chemical action are only different manifestations of *‘ energy 
of matter,” mutually convertible, but as indestructible as matter 
itself. 
Energy exists in two forms, dynamic or “‘kinetic energy,” or 
force manifesting itself to our senses as weight in motion, as 
sensible heat, or as an active electrical current; and “ potential 
energy,” or force in a dormant condition. In illustration of these 
two forms of energy, I will take the case of lifting a weight, say 
one pound one foot high. In lifting this weight ‘‘ kinetic mus- 
cular energy” has to be exercised in overcoming the force of 
gravitation of the earth. The pound weight when supported at 
the higher level to which it has been raised, represents potential 
energy to the amount of one unit or foot pound. This potential 
energy may be utilised in imparting motion to mechanism durin 
its descent, whereby a unit amount of ‘‘ Work” is accomplished. 
A pound of carbon then, when raised through the space of one 
foot from the earth, represents, mechanically speaking, a unit 
quantity of energy, but the same pound of carbon being separa- 
ted or lifted away from oxygen, to which it has a very powerful 
attraction, is capable of developing no less than 11,000,000 foot 
pounds or unit quantities of energy whenever the bar to their 
combination, namely excessive depression of temperature, is 
removed ; in other words, the mechanical energy set free in the 
combustion of one pound of pure carbon is the same as would be 
required to raise 11,000,000 pounds weight one foot high, or as 
would sustain the work which we call a horse power during 
5 hours 33 minutes. We thus arrive at once at the utmost limit 
of work which we can ever hope to accomplish by the combus- 
tion of one pound of carbonaceous matter, and we Shall presently 
see how far we are still removed in our steam engine practice from 
this limit of perfection.* 
The following illustrations will show the convertibility of the 
different forms of energy. If I let the weight of a hammer de- 
scend in rapid succession upon a piece of iron it becomes hot, 
and on beating a nail thus vigorously and skilfully for a minute it 
will be redhot. In this case the mechanical foree developed in 
the arm by the combustion of carbonaceous muscular fibre is eon- 
verted into heat. Again, in compressing the air in a fire syringe 
rapidly ignition of a piece of tinder is obtained. Again, in 
passing an electrical current through the platinum wire it is 
* In burning 1b. of carbon in the presence of free oxygen, carbonic acid 
is produced and 14,500 units of heat (1 1b, of water raised through 1° Fah.) 
are liberated. Fach unit of heat is convertible (as proyed by the deductions 
of Meyer and the actual measurements of Joule) into 774 units of force or 
mechanical energy ; hence 1 1b. of carbon represents really 14,500 % 774 = 
11,223,000 units of potential energy. , 
om 
